The foam-laid process for forming non-woven fibrous webs is basically disclosed in U.S. Pat. Nos. 3,716,449, 3,871,952, and 3,938,782 (the disclosures of which are incorporated by reference herein). The foam-laid process has a number of advantages over the water-laid process that is most conventionally used for making synthetic or cellulose fiber webs. The invention relates to a method and assembly for implementing the foam-laid process so as to improve aspects thereof.
While the foam process has a number of advantageous over the water-laid process in the production of fibrous non-woven webs, one of the practical disadvantageous thereof that has limited its commercialization to particular types of foams, is the relatively narrow width of the webs that have been produced from the foam-laid process in the past (e.g. typically in the range of 1-1.5 meters), compared with the width of the web in typical paper machines using the water-laid process which can be more than ten meters. Also, the production speed of the foam process has in the past been typically significantly under 100 meters per minute.
The main limitation in the width of the web and speed of operation in prior art systems implementing the foam process has been the pumps used to implement the process. The pumps are positive displacement pumps, such as screw pumps, twin screw pumps, twin rotor pumps, or the like, which have limited pumping capacity. Some of these positive displacement pumps are relatively insensitive to the material being pumped and, therefore, operate well in the production of fiber and gas containing fluids, which are, of course, characteristic of the foam process and it is for that reason that they are used. However, some of these pumps are easily worn out, are expensive, and easily damaged. Consequently, if the production is to be increased (as by increasing the size of the web produced by increasing the width of the wire or other foraminous element to more than 1.5 meters) several pumps must be used in parallel. This increases the expense of the assembly dramatically, and also introduces the risk of one of the pumps becoming damaged and thereby shutting down the entire process to repair or replace the damaged pump.
Even in situations where the prior art recognizes, during the practice of the foam process, the desirability to remove gas, some sort of separate degassing structure is provided, and pumping is done utilizing a conventional positive displacement pump. For example, in FIG. 3 of U.S. Pat. No. 4,944,843 degassing is provided by a centrifugal separator, but the foam which is passed through the foraminous element and is being removed through a duct is pumped using a positive displacement pump.
According to the present invention, disadvantages discussed above in a conventional foam process are overcome in a simple yet effective manner. By utilizing centrifugal pumps to handle the foam slurries (whether containing fibers or not) it is possible to increase the width of the wire (or other foraminous element) to more than two meters, and to increase the forming speed to more than 100 meters per minute (e.g. about 200-500 meters per minute). However, most centrifugal pumps are unsuitable for pumping the type of slurries that are handled in the foam process according to the invention. However, the invention utilizes degassing centrifugal pumps which have been found, according to the present invention, to be effective in implementing the foam process. While degassing centrifugal pumps, such as shown in U.S. Pat. Nos. 4,435,193 and 4,476,886, and Canadian Patent 1,128,368, have been used for many years in the pumping of medium consistency (e.g. about 6-18% solids) liquid fibrous slurries during the production of paper pulp and the like, their use in pumping the type of slurries encountered in the implementation of the foam process has not heretofore been recognized as practical, or as a solution to the long-standing problems in implementation of the foam process as set forth above.
According to one aspect of the present invention, a method of producing a non-woven web of fibrous material using a moving foraminous element (such as a single wire, twin wire, or any other conventional foraminous element) is provided. The method comprises the following steps: (a) Generating a first foam slurry of air, water, fibers, and a surfactant. (b) Centrifugally pumping the first foam slurry, into contact with the moving foraminous element. (c) Withdrawing substantially fiber-free foam from the foraminous element, while forming a non-woven web of fibrous material on the foraminous element, And (d) recycling at least part of the substantially fiber-free foam from step (c) for use in the practice of step (a).
Step (d) is preferably practiced in part by Centrifugally pumping the foam. Preferably steps (b) and (d) are practiced by partially degassing the foam during centrifugal pumping thereof (e.g. using a degassing centrifugal pump as basically described in U.S. Pat. Nos. 4,435,193 and 4,476,886, and Canadian Patent 1,128,368.
Steps (a) through (d) are preferably practiced using a moving foraminous element more than two meters wide (e.g. 2.1-10- meters wide), to produce as the non-woven fibrous web a web more than two meters wide. Also, steps (a) through (d) are preferably practiced to produce the non-woven web at a forming speed of more than about 100 meters per minute (e.g. more than about 200 meters per minute, such as about 200-500 meters per minute). Preferably centrifugal pumps are the only pumps used to pump either fibrous foam slurry or substantially fiber-free foam slurry in the practice of steps (a) through (d).
According to another aspect of the present invention a foam process assembly for producing a non-woven fibrous web is also provided. The assembly comprises the following components: A moving foraminous element on which a non-woven web may be formed. A source of a first foam slurry of air, water, fibers, and a surfactant. A first centrifugal pump for pumping the first foam slurry into contact with the moving foraminous element to form a non-woven web of fibrous material thereon, while a substantially fiber-free foam passes through the foraminous element. And a recycling system which returns at least part of the substantially fiber-free foam passing through the foraminous element to the source of the first foam slurry.
The moving foraminous element may be any conventional foraminous element, such as a single or double wire. The source of the first foam slurry may comprise any conventional source, such as a mixer/pulper, and/or a wire pit, and foam nozzles are typically utilized to facilitate the generate of the foam after pumping and before the foam is brought into contact with the foraminous element. The recycling system typically includes the wire pit, various conduits, and a second centrifugal pump (preferably a degassing pump as described above), and the first centrifugal pump is also preferably a degassing pump. The recycling system may comprise any conventional components, however. The recycling system typically includes the wire pit and the second centrifugal pump substantially fiber-free foam from the wire pit to the mixer/pulper. The foraminous element is preferably more than two meters wide, so as to produce a non-woven web more than two meters wide.
According to yet another aspect of the present invention a method of using a degassing centrifugal pump is provided. The method comprises the step of: pumping a foam slurry including at least gas, water and a surfactant with the degassing centrifugal pump, while simultaneously removing some gas from the slurry, during the production of a non-woven fibrous web by the foam process of web production. This step is typically practiced by pumping a foam slurry also including about 0.2-2.5%, by weight, fibers, and also by pumping a substantially fiber-free foam.
It is the primary object of the present invention to simply, yet effectively, enhance the implementation of the foam process for producing non-woven webs, including to increase the practical web width and/or forming speed. This and other objects of the invention will become from an inspection of the detailed description of the invention and from the appended claims.